The correlated discharge of action potentials by motor neurons is caused by common synaptic input that is delivered either by branched neurons or by rhythmic drive from supraspinal sources. The effect on motor unit activity is quantified as motor unit synchronization. Although technically challenging, the measurement of motor unit synchronization is a powerful technique that provides information about connections in the human CNS during voluntary muscle contractions. The two sources of common input can be distinguished by time-and frequency-domain analyses of the discharge times of pairs of motor units. This proposal focuses on the emerging concept that motor unit synchronization is a consequence of common rhythmic activity in the sensorimotor cortex. The presence of this rhythmicity in the descending drive onto motor neurons may enhance physiological tremor. We hypothesize that motor unit synchronization is caused by common oscillations present in the synaptic input received by motor neurons and that these increase with excitatory drive and impair the ability to perform steady contractions. Aim 1 is a modeling study that extends our existing motor unit model to include a current-based, threshold-crossing model of neuronal membrane potential coupled to a multi-compartment dendritic tree. With this model, we will explicitly control the synaptic inputs to the motor neurons with computer simulations and thereby determine the effects of common branched and rhythmic synaptic input on the correlated discharges of motor units. Interpretation of the experimental results (Aims 2 and 3) will be based on the outcomes of the simulations. Aim 2 will determine why the amount of motor unit synchronization is less at low forces compared with high forces. The correlated discharge of pairs of motor units will be measured at several discharge rates to distinguish among three possible factors: variation in the proportion of common synaptic input with the level of excitatory drive, greater synchronization for high threshold motor units, and changes in the source of common synaptic input. Aim 3 will examine the effect of motor unit synchronization on physiological tremor by comparing changes in the time- and frequency-domain measures of motor unit synchronization with the steadiness of performance across different types of muscle contractions. We expect to find that motor unit synchronization is largely due to common oscillations in the synaptic input onto motor neurons, that the proportion of common input changes both with the level of excitatory drive and the type of muscle contraction, and that this influences the steadiness of performance. The outcomes of this project will have direct application to the measurement and interpretation of motor unit synchronization in neurological patients.